Flexible bone screw

ABSTRACT

There is disclosed a device and method for stabilizing a bone. The device includes a flexible threaded portion operable to thread along a curved path in a bone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/191,904, filed Jul. 13, 2015, and U.S. Provisional Application No.62/238,780, filed Oct. 8, 2015, all of which are hereby incorporated byreference.

FIELD OF THE INVENTION

Examples of the invention relate generally to orthopedic devices for thesurgical treatment of bone and, more particularly, to the stabilizationof bones with an intramedullary device.

BACKGROUND

Orthopedic medicine provides a wide array of implants that can beattached to bone to repair fractures. External fixation involves theattachment of a device that protrudes out of the skin, and thereforecarries significant risk of infection. Many fractures in long bones canbe repaired through the use of bone plates, which are implanted andattached to lie directly on the bone surface. The bone plate thenremains in the body long enough to allow the fractured bone to healproperly. Unfortunately, such bone plates often require the surgicalexposure of substantially the entire length of bone to which the plateis to be attached. Such exposure typically results in a lengthy andpainful healing process, which must often be repeated when theimplantation site is again exposed to allow removal of the plate. Thereis a need in the art for implants and related instruments that do notrequire such broad exposure of the fractured bone, while minimizing theprobability of infection by avoiding elements that must protrude throughthe skin as the bone heals.

SUMMARY

Examples of the invention provide devices and methods for stabilizingfirst and second bone portions relative to one another.

In one example of the invention, a bone screw includes an elongate bodyhaving a distal portion, a mid-portion and a proximal portion spacedlongitudinally relative to a longitudinal axis. The distal portionincludes a helical thread having a major diameter and a minor diameter.The mid-portion has a non-threaded outer surface with an outer diameter.The mid-portion outer diameter is equal to or greater than the threadmajor diameter and the distal threaded portion is operable to bend as itis threaded into a bone to follow a curved path.

In another example of the invention, a plurality of bone screws includesfirst and second bone screws each having an elongate body having adistal portion, a mid-portion and a proximal portion spacedlongitudinally relative to a longitudinal axis. The distal portion ofeach screw has a helical thread formed on it having a minor diameter, apitch and a major diameter. The second bone screw thread minor diameteris equal to the first bone screw thread minor diameter, the second bonescrew thread pitch is equal to the first bone screw thread pitch, andthe second bone screw thread major diameter is greater than the firstbone screw thread major diameter.

In another example of the invention, a bone screw includes an elongatebody having a distal portion and a proximal portion spacedlongitudinally relative to a longitudinal axis. The distal portion has ahelical thread formed on it. The thread has a major diameter and a minordiameter. The distal threaded portion is operable to bend as it isthreaded into a bone to follow a curved path. The threaded distalportion has a bending stiffness that is less than the bending stiffnessof the proximal portion.

In another example of the invention, a method of inserting a screw intoa bone includes selecting a bone screw comprising an elongate bodyhaving a distal portion, a mid-portion and a proximal portion spacedlongitudinally relative to a longitudinal axis, the distal portionhaving a helical thread formed thereon, the thread having a majordiameter and a minor diameter, the mid-portion having a non-threadedouter surface with an outer diameter, the mid-portion outer diameterbeing equal to or greater than the thread major diameter; forming acurved tunnel in a bone; and turning the threaded distal portion of thebone screw into the tunnel so that the threaded distal portion bends tofollow the curve of the tunnel.

In another example of the invention, a method of inserting a screw intoa bone includes selecting a bone screw from a plurality of bone screwsincluding a first bone screw comprising an elongate body having a distalportion, a mid-portion and a proximal portion spaced longitudinallyrelative to a longitudinal axis, the distal portion having a helicalthread formed thereon, the thread having a minor diameter, a pitch and amajor diameter; and a second bone screw comprising an elongate bodyhaving a distal portion, a mid-portion and a proximal portion spacedlongitudinally relative to a longitudinal axis, the distal portionhaving a helical thread formed thereon, the thread having a minordiameter, a pitch and a major diameter; the second bone screw minordiameter being equal to the first screw minor diameter, the second bonescrew pitch being equal to the first screw pitch, and the second bonescrew major diameter being greater than the first screw major diameter;forming a tunnel in a bone; turning the selected bone screw into thetunnel.

BRIEF DESCRIPTION OF THE DRAWINGS

Various examples of the invention will be discussed with reference tothe appended drawings. These drawings depict only illustrative examplesof the invention and are not to be considered limiting of its scope.

FIG. 1 is a side elevation view of a screw according to one example ofthe invention;

FIG. 2 is a detail view of the screw of FIG. 1;

FIG. 3 is a detail view of the screw of FIG. 1;

FIG. 4 is an end view of the screw of FIG. 1;

FIGS. 5-7 are side elevation views of a set of differently sized screwslike that of FIG. 1;

FIGS. 8-10 are partial sectional views showing the insertion of thescrew of FIG. 1 into bone;

FIGS. 11-35 illustrate a surgical procedure utilizing the bone screw ofFIG. 1;

FIG. 36 is a perspective view of a screw according to one example of theinvention;

FIG. 37 is a top plan view of the screw of FIG. 36;

FIG. 38 is a side elevation view of the screw of FIG. 36;

FIG. 39 is an end view of the screw of FIG. 36;

FIG. 40 is a sectional view taken along line 40-40 of FIG. 37;

FIG. 41 is an exploded sectional view taken along line 40-40 of FIG. 37;

FIG. 42 is a cross sectional view of a screw similar to the screw ofFIG. 36 according to one example of the invention; and

FIG. 43 is an exploded cross sectional view of the screw of FIG. 42.

DESCRIPTION OF THE ILLUSTRATIVE EXAMPLES

The term “transverse” is used herein to mean not parallel. FIGS. 1-4depict a bone screw 100 having an elongate body 102 with a distalportion 104, a mid-portion 106 and a proximal portion 108 spacedlongitudinally relative to a longitudinal axis 110. The distal portion104 includes a helical thread 112 having a major diameter 114, a minordiameter 116, and a pitch 128. The mid-portion 106 has a non-threadedouter surface 118 with an outer diameter 120. In the illustrativeexample of FIGS. 1-4, the mid-portion outer diameter 120 is equal to orgreater than the thread major diameter 114. The distal threaded portion104 is operable to bend as it is threaded into a bone to follow a curvedpath. For example, the bending stiffness of the distal threaded portion104 is such that it will bend to follow a curved path in human bone.Such a curved path may be defined, for example, by a curved hole in thebone, a guide wire, or a natural bone feature such as a non-linearintramedullary canal bounded by cortical bone. This is distinct fromprior art screws which if started on a curved path in human bone would,when advanced, continue in a straight line and thus deviate from thecurved path and form their own, straight, path through the bone.Preferably the bending stiffness of the threaded distal portion 104 islower than the bending stiffness of the mid-portion 106. The relativelylower bending stiffness of the threaded distal portion 104 causes thethreaded distal portion to bend to follow a curved path while therelatively higher bending stiffness of the mid-portion causes themid-portion to remain straight to stabilize first and second boneportions relative to one another at a bone interface such as at afracture, osteotomy, or fusion site. The difference in bending stiffnessbetween the threaded distal portion 104 and the mid-portion 106 may beachieved in different ways. For example, the threaded distal portion 104and the mid-portion 106 may be made of different materials and/or mayhave different sectional moduli. In the illustrative example of FIGS.1-4, the threaded distal portion 104 and the mid-portion 106 havedifferent sectional moduli. The threaded distal portion minor diameter116 is less than the outer diameter 120 of the mid-portion 106 and thethreaded distal portion major diameter is less than or equal to theouter diameter 120 of the mid-portion 106. Preferably, the ratio of thebending stiffness of the mid-portion 106 to the bending stiffness of thethreaded distal portion 104 is in the range of 1.5:1 to 100:1. Morepreferably, the ratio is in the range of 2:1 to 20:1. For example,screws suitable for internal fixation of a clavicle fracture and thatfall within these ranges may have a major diameter 114 in the range of4-6.5 mm, a minor diameter 116 in the range of 2.5-3.5 and a cannulation101 with a diameter in the range of 1-2 mm. Preferably, the screw 100 ismade of a polymer.

Table 1 compares the calculated load required to bend a cantileveredtube of 3 mm outside diameter and 1.5 mm inside diameter around a radiusof 50 mm and an arc length of 26 mm. The titanium and stainless steelalloys are predicted to have a required load approximately 10 times thatof the PEEK and PLLA. These loads would be greater than the bone couldwithstand and a threaded device made of those materials would not followa curved path in the bone but would instead cause the bone to fail. Inthe case of the highly cold worked stainless steel, even if the bonecould withstand the load, the screw would fail since the minimum bendradius before failure of the screw is greater than 50 mm.

TABLE 1 Load at 50 mm bend radius Yield Failure Yield Failure FlexuralStress Stress Strain Strain Modulus Load Material (MPa) (MPa) (%) (%)(MPa) (N) PEEK 100 115 2.5% 20% 4 9.8 ASTM F2026 PLLA 90 100 2.6% 25%3.5 8.7 Ti—6Al—4V 880 990 0.8% 14% 114 91.7 ELI ASTM F136 316LVM 14681696 0.7% 3% 197 Not Stainless Steel possible ASTM F899

Another way to quantify the bending stiffness of the threaded distalportion 104 is by the amount of torque required to turn the threadeddistal portion 104 into a curved bone hole having a specified radius ofcurvature. For example, the threaded distal portion 104 preferablyrequires a torque less than 20 in-lbs to turn the distal threadedportion 104 into a bone to follow a curved path having a radius ofcurvature of 50 mm. More preferably the required torque is less than 10in-lbs. More preferably the required torque is less than 5 in-lbs. Morepreferably the required torque is approximately 2 in-lbs.

Table 2 compares the measured torque required to advance a threaded tube25 mm into a 50 mm threaded radius formed in a rigid test block. Thetubes were all machined to the same geometry but of different materials.The thread major diameter was 4.25 mm, the minor diameter was 3.0 mm andthe inner diameter of the tube was 1.5 mm. A rigid block was preparedhaving a curved, threaded path. Such a path has a pitch that is wider onthe outside of the curve and a pitch that is narrower on the inside ofthe curve corresponding to the shape of the screw thread when it iscurved. Multiple samples of each screw were inserted into the block overan arc length of 25 mm. The maximum torque for each revolution wasmeasured and it was found that the torque increased for each revolution.In Table 2, the range is the range of torque values from the first tothe last revolution. The average is the average of the torque values forall revolutions. The peak is the highest torque value and in all casesoccurred in the last revolution. However, the torque values for eachmaterial were relatively constant over the last few revolutions. Thetitanium and stainless steel alloys had measured torque valuesapproximately 10 times that of the PEEK. These tests were conductedusing a threaded block made of tool steel with a strength greater thanthat of the materials being tested in order to compare the torquevalues. As pointed out relative to Table 1, the loads generated from themetal implants would be greater than the bone could withstand and athreaded device as described herein made of these metals would notfollow a curved path in the bone but would instead cause the bone tofail.

TABLE 2 Torque to thread around rigid 50 mm radius Material Range(in-lbs) Average (in-lbs) Peak (in-lbs) PEEK  0-2.0 1.4 2.0 ASTM F2026Ti—6Al—4V ELI 0.7-25 16 25 ASTM F136 316LVM 0.5-20 13 20 Stainless SteelASTM F899

In addition to bending stiffness advantages, having the threaded distalportion major diameter less than or equal to the outer diameter 120 ofthe mid-portion 106 allows the distal threaded portion 104 to passthrough a passage in a bone that will be a sliding or press fit with themid-portion 106. A screw so configured, as shown in the illustrativeexample of FIGS. 1-4, can have an intramedullary canal fillingmid-portion 106 providing solid support to a bone interface and arelatively bendable distal threaded portion 104 following a curved pathsuch as for threading into a distal portion of a curved bone to securethe screw in the bone.

The proximal portion 108 may be identical to the mid-portion 106.Alternatively, the proximal portion may have a positive driverengagement feature (not shown) such as internal or external non-circularsurfaces, profiles, or holes. For example, an internal or externalslotted, threaded, triangular, square, hexagonal, hexalobular, or otherdrive feature may be provided. In addition, as shown in the illustrativeexample of FIGS. 1-4, the proximal portion 108 may include an optionalexternal helical thread 122 able to engage a bone portion to provideproximal fixation of the screw. For example, the proximal thread 122 mayhave a major diameter 124, a minor diameter 126, and a pitch 130 whereinthe proximal thread minor diameter 126 is equal to the mid-portion outerdiameter 120. In the illustrative example of FIGS. 1-4, the mid-portionouter diameter 120 is equal to the proximal thread minor diameter 126and the distal thread major diameter 114. The proximal portion mayalternatively, or in addition, receive a locking member such as a pin orscrew transverse to the longitudinal axis to lock a proximal boneportion to the nail. The locking member may be drilled through theproximal portion. Preferably, the proximal portion has one or moretransverse holes formed through it for receiving the locking member.

The distal and proximal thread pitches 128, 130 may advantageously bethe same or different depending on the application. For example, tostabilize a fracture, the screw 100 may be inserted into a bone acrossthe fracture so that the distal thread 112 is engaged with bone distalto the fracture and the proximal thread 122 is engaged with boneproximal to the fracture. If the bone portions on either side of thefracture are reduced to a desired final position prior to inserting thescrew 100, then it is advantageous for the thread pitches 128, 130 to beequal so that insertion of the screw does not change the relativepositions of the bone portions. If on the other hand, it is desirable tomove the bone portions relative to one another by the action ofinserting the screw then it is advantageous for the pitches 128, 130 tobe different. For example, to move the bone portions closer together toreduce the fracture, the distal thread pitch 128 may be made greaterthan the proximal thread pitch 130 so that with the distal thread 112engaged distally and the proximal thread 122 engaged proximally, furtheradvancing the screw causes the distal bone portion to move proximallyrelative to the screw faster than the proximal bone portion movesproximally and thus move the bone portions closer together.Alternatively, to move the bone portions further apart to distract thefracture, the distal thread pitch 128 may be made smaller than theproximal thread pitch 130 so that with the distal thread 112 engageddistally and the proximal thread 122 engaged proximally, furtheradvancing the screw causes the distal bone portion to move proximallyrelative to the screw more slowly than the proximal bone portion movesproximally and thus move the bone portions further apart. Preferably,the bone screw 100 has a through bore, or cannulation 101, coaxial withthe longitudinal axis 110 to permit the bone screw 100 to be insertedover a guide wire.

The bone screw 100 of FIGS. 1-4, may advantageously be provided in a setcontaining a plurality of bone screws as shown in the illustrativeexample of FIGS. 5-7. For example, it is advantageous in a surgicalprocedure to minimize the number of steps and the amount of time neededto complete the procedure. In a bone fixation procedure, a surgeon oftenmakes an initial sizing decision based on medical imaging. During theprocedure, it may become expedient to change the predetermined sizebased on observation of the surgical site or the fit of trial implantsor instruments. For example, a surgeon may determine initially that asmaller bone screw is appropriate. However, during preparation of thesite, the surgeon may determine that a larger screw will better grip thebone or fill, for example, a canal in the bone. The illustrative set ofbone screws shown in FIGS. 5-7 facilitates changing between sizes. Eachscrew 140, 150, 160 in the set has a minor diameter 142, 152, 162, amajor diameter 144, 154, 164, and a pitch 146, 156, 166. The minordiameters 142, 152, 162 are equal to one another so that a singlediameter drill will provide an initial bore hole appropriate for all thescrews in the set. The pitches 146, 156, 166 are equal to one another sothat all of the screws in the set will threadably engage a helicalthread of the same pitch. The major diameters 144, 154, 164 may increaseto provide progressively more bone purchase or, for example, to spanincreasing larger intramedullary canals. For example, with the set ofscrews of the illustrative example of FIGS. 5-7, a surgeon may drill ahole equal to the minor diameters 142, 152, 162 and then tap the holewith a tap corresponding to the thread of the smallest major diameterscrew 140. The tactile feedback received by the surgeon as the tap isinserted will indicate to the surgeon if the thread major diameter issufficient to provide a desired level of bone engagement. For example,the surgeon can feel if the tap is engaging the cortical walls of anintramedullary canal or if the tap is in softer cancellous bone. If thesurgeon determines that greater engagement is desired, the surgeon cannext tap the hole with a tap corresponding to the thread of the nextlarger major diameter screw 150. Since the minor diameters 142, 152, 162and thread pitches 146, 156, 166 are the same for all of the screws inthe set, the next tap will thread into the previously tapped hole andincrease the bone thread major diameter without damaging the bonethread. Once the desired bone engagement is achieved, the surgeon maythen insert the desired screw 140, 150, 160. If in tapping the largermajor diameter thread, the surgeon determines that the bone is providingtoo much resistance, the surgeon may revert to the smaller sized screwsince the threads are still compatible. Alternatively to using aseparate tap, the screw threads may be configured as self-tapping sothat the screws may be threaded directly into the bored hole.

In addition to the sizing advantages of having the same minor diameter142, 152, 162 across a family of screws, it is also advantageous becausethe distal threaded portion of each screw will have a similar bendingstiffness to each of the other screws 140, 150, 160 since the continuouswall of the minor diameter contributes much more to the bendingstiffness than the helical thread itself. This similar bending stiffnessmeans that they can be inserted around a similar bending radius with asimilar torque.

In the illustrative example of FIGS. 5-7, each screw 140, 150, 160 has amid-portion diameter 148, 158, 168 equal to the corresponding majordiameter 144, 154, 164. The increasing mid-portion diameters provideprogressively less flexible mid-portions across the set of screws and,for example, canal filling for increasingly larger bones if used in theintramedullary canal. If the screws incorporate the optional increasingmid-portion diameter as shown, then it is desirable to re-drill themid-portion of the bone hole to accommodate the mid-portion when anincrease in screw size is desired. However, the distal, threaded portionof the bone hole does not need to be re-drilled so the screw threadswill not be damaged by drilling.

Alternatively to, or in addition to, the threaded distal portion 104 andmid-portion 106 having different sectional moduli, the threaded distalportion 104 and mid-portion 106 may have different material propertiessuch as two different materials or different conditions of the samematerial to produce a difference in bending stiffness between them.

In the illustrative example of FIGS. 36-41, a screw 170 has separatefirst and second members 172, 174 permanently joined together. The firstmember 172 includes an elongate body 176 with a proximal end 178, adistal end 180, a longitudinal axis 182, and an axial through bore 184.The proximal end 178 of the first member includes a pair of transversethrough bores 181, 183. Each transverse bore 181, 183 defines alongitudinal axis and the axes form an angle 185 between them about thelongitudinal axis 182 as best seen in FIG. 39. Providing more than onetransverse through bore increases options for attaching the screw tobone fragments and options for fixation direction. Both bores may beused for fixation or the one that is most conveniently located.Preferably the angle 185 is in the range of 0 to 90 degrees. Morepreferably the angle 185 is in the range of 20 to 90 degrees. In theillustrative example of FIGS. 36-41, the angle 185 is 45 degrees. Theproximal end 178 also includes opposed flats 187 for engaging a driverin torque transmitting relationship. An internal thread 189 within thebore 184 is engageable with, e.g., a threaded draw bar to secure thefirst member to a driver.

The second member 174 includes an elongate body 186 with a proximal end188, a distal end 190, a longitudinal axis 192, an external helicalthread 194, and an axial through bore 196. The distal end 180 of thefirst member 172 and the proximal end 188 of the second member 174 mayhave complementary geometries to aid in joining them. In theillustrative example of FIGS. 36-41, the distal end 180 of the firstmember has a stepped conical taper and the proximal end 188 of thesecond member has a corresponding stepped conical socket 198. The matingsurfaces may be any suitable shape as determined by the materials andjoining technique including but not limited to plug and socket joints(as shown), scarf joints, butt joints, dovetail joints, finger joints,and lap joints. The joint may be reinforced with a third component suchas an adhesive, pin, or key. The joint may be formed by mechanicalinterlock, chemical bonding, molding, welding or other suitable joiningprocess. The final assembled screw 170, has a distal portion 191, amid-portion 193, and a proximal portion 195 and may have the threadforms, diameters, and relationships as described relative to theexamples of FIGS. 1-7.

The first and second components 172, 174 may be made of differentmaterials or different conditions of the same material. For example,they may be made of polymers, metals, or ceramics. Metals may includestainless steel alloys, titanium, titanium alloys, cobalt-chromium steelalloys, nickel-titanium alloys, and/or others. Polymers may includenonresorbable polymers including polyolefins, polyesters, polyimides,polyamides, polyacrylates, poly(ketones), fluropolymers, siloxane basedpolymers, and/or others. Polymers may include resorbable polymersincluding polyesters (e.g. lactide and glycolide), polyanhydrides,poly(aminoacid) polymers (e.g. tyrosine based polymers), and/or others.Other possible materials include nonresorbable and resorbable ceramics(e.g. hydroxyapatite and calcium sulfate) or biocompatible glasses. Theymay be made of homogenous materials or reinforced materials. They may bemade of crystallographically different materials such as annealed versuscold worked. It is preferable for the mid portion 193 to have a higherbending stiffness than the distal portion 191 and the distal portionshould have a bending stiffness low enough for it to be inserted along acurved path in bone.

In a first example, the first component may be made of a metal with arelatively high degree of cold work and the second component of a metalwith a relatively low amount of cold work such as for example annealedand cold worked stainless steel. The components may be joined forexample by welding. However, as discussed relative to Table 1, mostmetals are far too stiff to allow threading along a curved path in abone within suitable torsional loads.

Preferably the distal portion is made of a polymer. In a second example,the first component is made of a metal, such as stainless steel or atitanium alloy, and the second component is made of a polymer such aspolyetheretherketone (PEEK) or a polylactide polymer (e.g. PLLA). Thecomponents may be joined such as for example by threading them together.

Preferably both components are made of polymers. In a third example, thefirst and second components are both made of non-resorbable polymers.For example, the first component may be made of fiber reinforced PEEK(e.g. Invibio PEEK-Optima™ Ultra-Reinforced) and the second componentmay be made of neat (unreinforced) PEEK (e.g. Invibio PEEK-Optima™Natural). The fiber reinforced PEEK is strong while the neat PEEK isrelatively flexible allowing it to be easily threaded around a curvedpath even while having a relatively large bone filling diameter. Thecomponents may be joined, e.g. by molding the components as a continuousmatrix with first component fiber reinforcement and second componentneat polymer with polymer chains extending across the joint interface.In the example of FIGS. 36-41, the second component is relatively moretransparent to laser radiation than the first component and the partsare joined by laser welding at the conical interface. The laser energypasses relatively easily through the second component and is absorbed bythe first component so that localized heating at the conical interfacetakes place causing the polymer constituent of the two components tofuse together.

In a fourth example, the mid-portion and distal portion are made ofresorbable polymers. For example, the mid-portion may be made of a glassfiber reinforced PLLA (e.g. Corbion-Purac FiberLive™) and the distalportion may be made of neat PLLA.

Alternatively, the first member 172 and second member 174 may form onecontinuous part with different properties between first and secondportions. The difference in properties may be achieved, for example, bydifferent processing (e.g. thermal processing) or blending materials.For example, different polymers may be combined in a single injectionmold cavity and formed together. The polymers may be blended so thatthere is a transition between them. In another example, stiffeningand/or strengthening material, e.g. fibers, whiskers, and/or granules,may be selectively incorporated in, e.g., the first portion.

FIGS. 42 and 43 illustrate an example of a screw 270 similar to that ofFIGS. 36-41 except that the first member 272 is not cannulated, thefirst member 272 extends the full length of the second member 274, andthe transverse holes 281, 283 are coplanar. The screw 270 may beassembled as with the prior example including by using complimentaryscrew threads in the proximal region of the second member 274 and midportion of the first member 272 as indicated by reference number 250.The screw 270 of the example of FIGS. 42 and 43 may be include any ofthe materials and features described relative to the prior examples. If,for example, the first member 272 is made of a radiographically moreopaque material than the second member 274, then the first member willprovide a radiographic marker over the entire length of the screw 270that may be radiographically visualized during and after surgery toconfirm screw placement. For example, a metal first component andpolymer second component would provide for radiographic visualization ofthe metal first component. It has been found by the present inventorsthat the bending stiffness of the distal end of the screw is notmaterially changed by eliminating the axial through bore of the firstcomponent and is essentially unchanged when the bending stiffness of aguide wire is accounted for which was optionally used with the previouscannulated screw examples. The guide wire is not necessary inasmuch asthe screw 270 will follow a curved hole prepared to receive it. Thetransverse holes 181, 183 may be provided in any number or not at all asdesired but it has been found that one is sufficient and two providesthe user with additional fixation choice.

FIGS. 8-10 illustrate a bone screw according to the one example of theinvention, such as bone screw 100, being inserted into first and secondbone portions 200, 202 having a bone interface 204 between them. A firstor proximal bore 206 is formed in the first bone portion 200, across thebone interface 204, and into the second bone portion 202. A second ordistal bore 208 extends distally from the proximal bore 206 defining acurved path 210. The screw 100 is advanced through the proximal bore 206until the distal screw threads engage the distal bore 208 as shown inFIG. 9. Further advancing the screw 100 causes it to bend to follow thecurved path 210 as shown in FIG. 10. Having a straight portion of thepath, and thus the straight mid portion of the screw 100, spanning thebone interface results in a zero stress and strain state at the boneinterface which prevents separation of the bone portions 200, 202 at theinterface 204.

FIGS. 11-35 depict an illustrative method of using the screw of FIGS.1-4 to fix a fractured clavicle. A patient is placed in a beach chairposition with the head rotated away from the operative side. A bolsteris placed between the shoulder blades and head allowing the injuredshoulder girdle to retract posteriorly. A C-arm is positioned to enableanterior-posterior (AP) and cephalic views of the operative site. A 2-3cm incision 300 is made at the fracture site along Langer's Linesrunning perpendicular to the long axis of the clavicle to expose thefracture site (FIG. 10). The platysma muscle is freed from the skin andsplit between its fibers. The middle branch of the supraclavicular nerveis identified and retracted.

The medial end 302 of the lateral fragment 304 of the fractured clavicleis elevated from the fracture site incision (FIG. 12).

A K-wire 306, e.g. a 1.4 mm K-wire, is drilled into the canal of thelateral fragment 304 and advanced through the dorsolateral cortex 308and out through the skin (FIG. 13).

A wire driver is attached to the lateral portion of the K-wire and usedto back the wire out until it is lateral to the fracture 310 (FIG. 14).Bone clamps are used at the incision site to reduce the fracture andclamp the bone fragments in position. Proper reduction is confirmed withAP and cephalic radiographic views.

The K-wire 306 is advanced until it is preferably at least 20 mm medialto the fracture (FIG. 15).

A first dilator 312, e.g. a 3.2 mm dilator, is placed over the K-wireand advanced until it contacts the bone (FIGS. 16-17).

A second dilator 314, e.g. a 4.5 mm dilator, is placed over the firstdilator 312 and advanced until it contacts the bone (FIG. 18).

A drill guide 316 is placed over the second dilator 314 and advanceduntil it contacts the bone (FIG. 19).

The first dilator 312 is removed and a first lateral drill 318,corresponding to the minor diameter of the distal screw threads, e.g. a3.2 mm drill, is advanced over the K-wire into the bone, preferably atleast 20 mm medial to the fracture. A drill depth mark readable adjacentthe drill guide may be noted as a reference for implant sizing (FIG.20).

The K-wire is removed and replaced with a flexible guide wire 320, e.g.a nitinol guide wire, sized to fit within the screw cannulation, e.g. a1.4 mm guide wire. The flexible guide wire 320 is advanced through thefirst lateral drill and further along the intramedullary canal of themedial bone fragment and will curve to follow the intramedullary canalto define a curved path in the bone. Preferably, the guide wire isadvanced approximately 30 mm medial to the tip of the first lateraldrill 318 (FIG. 21).

The first lateral drill 318 is removed and a flexible shaft reamer 322,corresponding to the minor diameter of the distal screw threads, isguided over the flexible guide wire 320 to ream the medial portion ofthe curved path (FIG. 22) The flexible reamer 322 and second dilator 314are then removed.

A second lateral drill 324, having a diameter corresponding to thediameter of the mid-portion of the screw, e.g. a 4.5 mm drill, is guidedover the flexible guide wire to enlarge the bone hole laterally toreceive the mid-portion and proximal portion of the screw 100. Thesecond lateral drill 324 is advanced the same distance as the firstlateral drill (FIG. 23). The drilling step may be monitored in A/P andcephalic views with the C-arm to avoid perforating the bone cortex asthe second lateral drill 324 is advanced into the medial bone fragment326.

A flexible tap 328, having cutting threads corresponding to the distalthreads of the screw 100 is guided over the flexible guide wire to cutthreads into the medial bone fragment along the curved path (FIG. 24).The tap may serve as a trial implant and provides tactile feedbackregarding the fit of the implant in the bone. If it is determined that alarger screw is desirable, subsequent larger second drills may be usedto re-drill the lateral straight portion and subsequent larger flexibletaps may be used to increase the distal thread major diameter withouthaving to re-ream the medial curved portion of the bone hole. Once adesired level of thread purchase and canal filling are achieved, a depthmark readable adjacent the drill guide may be noted as a reference forthe required implant length. If a screw 100 with a proximal threadedportion is used, a lateral tap may be used to tap the lateral bonefragment to receive the proximal threads.

The screw 100 is attached to an inserter 330 and guided over theflexible guide wire until it is fully seated in the prepared threads inthe medial bone fragment (FIGS. 25 and 26). Optionally, the screw 100may be axially driven with a mallet through the lateral bone fragmentuntil just short of the distal thread engagement. The screw 100 may thenbe threaded into full engagement with the prepared threads in the medialfragment. Radiographic visualization may be used to ensure that thefracture is fully reduced and anatomically aligned in length androtation.

If a proximally threaded screw has not been used, or if additionalfixation is otherwise desired, cross fixation may be used. For example,a cross fixation guide 340 may be engaged with the implant inserter 330(FIG. 27). The cross fixation guide may include a knob 342 thatthreadingly engages the implant inserter 330 and a cross fixation guidesleeve 344 that abuts the lateral bone fragment adjacent the bone holeentrance. Rotating the knob 342 moves the cross fixation guide sleeve344 and implant inserter 330 axially relative to one another. With thecross fixation guide sleeve 344 abutting the lateral bone fragment 304,the implant inserter, implant, and medial bone fragment 326 will bedrawn laterally and the lateral bone fragment 304 will be pressedmedially to apply compression across the fracture.

Inner and outer drill sleeves 346, 348 are advanced through the guide340 until they abut the bone (FIG. 28). In the case of a screw such asscrew 170 having one or more preformed transverse bores, the crossfixation guide may have one or more targeting holes positioned to alignwith the one or more transverse bores. In the case of a screw such asscrew 100 not having preformed transverse bores, cross fixation may beinserted directly through the screw 100 forming a transverse boreintraoperatively.

For example, a cross fixation wire 350 may be guided through the drillsleeves, through the near cortex, through the mid or proximal portionsof the screw, and into the far cortex of the lateral bone fragment (FIG.29). If wire cross fixation is adequate, the cross fixation guide may beremoved and the wire may be trimmed flush with the bone surface.

However, if screw cross fixation is desired, a screw depth gauge 352 maybe placed over the cross fixation wire to measure the projecting portionof the guide wire to determine the required screw length for bi-corticalfixation (FIG. 30).

A countersink tool 354 may be used to create a countersink for a crossfixation bone screw 356 (FIG. 31).

The appropriate length cross fixation screw 356 may then be guided overthe cross fixation wire 350 and seated into the bone (FIG. 32). Thesesteps may be repeated to place additional screws if desired.

FIGS. 33 and 34 illustrate the location of the screw 100 and crossfixation screws 356 relative to the lateral and medial bone fragments.

FIG. 35 illustrates the cross fixation screws 356 in the screw 100without the bone to obscure the view. Preferably the screw 100 is madeof a relatively soft material, e.g. a polymer, that facilitatesarbitrary placement of the cross fixation screws at any desiredlocation.

What is claimed is:
 1. A bone screw comprising an elongate body having adistal portion, a mid-portion and a proximal portion spacedlongitudinally relative to a longitudinal axis, the distal portionhaving a helical thread formed thereon, the thread having a majordiameter and a minor diameter, the mid-portion having a non-threadedouter surface with an outer diameter, the mid-portion outer diameterbeing equal to or greater than the thread major diameter, the distalthreaded portion being operable to bend as it is threaded into a bone tofollow a curved path.
 2. The bone screw of claim 1 further comprising ahelical thread formed on the proximal portion.
 3. The bone screw ofclaim 2 wherein the distal thread has a distal thread pitch and theproximal thread has a proximal thread pitch, the proximal thread pitchbeing greater than the distal thread pitch.
 4. The bone screw of claim 2wherein the distal thread has a distal thread pitch and the proximalthread has a proximal thread pitch, the proximal thread pitch beingequal to the distal thread pitch.
 5. The bone screw of claim 2 whereinthe distal thread has a distal thread pitch and the proximal thread hasa proximal thread pitch, the proximal thread pitch being less than thedistal thread pitch.
 6. The bone screw of claim 2 wherein the proximalthread has a major diameter and a minor diameter, the proximal threadminor diameter being equal to the mid-portion outer diameter.
 7. Thebone screw of claim 1 wherein the threaded distal portion has a bendingstiffness and the mid-shaft portion has a bending stiffness, thethreaded distal portion bending stiffness being lower than the mid-shaftportion bending stiffness.
 8. The bone screw of claim 7 wherein theratio of the mid-shaft portion bending stiffness to the distal portionbending stiffness is in the range of 2:1 to 20:1.
 9. The bone screw ofclaim 1 wherein the torque required to turn the distal threaded portioninto a bone to follow a curved path having a radius of curvature of 50mm is less than 5 inch-lbs.
 10. The bone screw of claim 1 wherein atleast the distal portion comprises a polymeric material.
 11. A pluralityof bone screws comprising: a first bone screw comprising an elongatebody having a distal portion, a mid-portion and a proximal portionspaced longitudinally relative to a longitudinal axis, the distalportion having a helical thread formed thereon, the thread having aminor diameter, a pitch and a major diameter; and a second bone screwcomprising an elongate body having a distal portion, a mid-portion and aproximal portion spaced longitudinally relative to a longitudinal axis,the distal portion having a helical thread formed thereon, the threadhaving a minor diameter, a pitch and a major diameter; the second bonescrew thread minor diameter being equal to the first bone screw threadminor diameter, the second bone screw thread pitch being equal to thefirst bone screw thread pitch, and the second bone screw thread majordiameter being greater than the first bone screw thread major diameter.12. The plurality of bone screws of claim 11 further comprising: a thirdbone screw comprising an elongate body having a distal portion, amid-portion and a proximal portion spaced longitudinally relative to alongitudinal axis, the distal portion having a helical thread formedthereon, the thread having a minor diameter, a pitch and a majordiameter; the third bone screw thread minor diameter being equal to thesecond bone screw thread minor diameter, the third bone screw threadpitch being equal to the second bone screw thread pitch, and the thirdbone screw thread major diameter being greater than the second bonescrew thread major diameter.
 13. The plurality of bone screws of claim11 wherein the first bone screw mid-portion has a diameter and thesecond bone screw mid-portion has a diameter, the second bone screwmid-portion diameter being greater than the first bone screw mid-portiondiameter.
 14. The plurality of bone screws of claim 11 wherein thedistal threaded portion of each screw is operable to bend as it isthreaded into a bone to follow a curved path.
 15. The plurality of bonescrews of claim 11 wherein at least the distal portion of each screwcomprises a polymeric material.
 16. The plurality of bone screws ofclaim 11 wherein each screw further comprises a helical thread formed onthe proximal portion.
 17. The plurality of bone screws of claim 16wherein the distal thread of each screw has a distal thread pitch andthe proximal thread of each screw has a proximal thread pitch, theproximal thread pitch of each screw being greater than the distal threadpitch of the same screw.
 18. The plurality of bone screws of claim 16wherein the distal thread of each screw has a distal thread pitch andthe proximal thread of each screw has a proximal thread pitch, theproximal thread pitch of each screw being equal to the distal threadpitch of the same screw.
 19. The plurality of bone screws of claim 16wherein the distal thread of each screw has a distal thread pitch andthe proximal thread of each screw has a proximal thread pitch, theproximal thread pitch of each screw being less than the distal threadpitch of the same screw.
 20. The plurality of bone screws of claim 16wherein the proximal thread of each screw has a major diameter and aminor diameter and the mid-portion of each screw has an outer diameter,the proximal thread minor diameter of each screw being equal to themid-portion outer diameter of the same screw and the distal thread majordiameter of each screw being equal to the mid-portion outer diameter ofthe same screw.
 21. The plurality of bone screws of claim 11 wherein thetorque required to turn the distal threaded portion of each screw intothe bone to follow a curved path having a radius of curvature of 50 mmis less than 5 inch-lbs.
 22. A bone screw comprising an elongate bodyhaving a distal portion and a proximal portion spaced longitudinallyrelative to a longitudinal axis, the distal portion having a helicalthread formed thereon, the thread having a major diameter and a minordiameter, the distal threaded portion being operable to bend as it isthreaded into a bone to follow a curved path, the threaded distalportion having a bending stiffness and the proximal portion having abending stiffness, the bending stiffness of the proximal portion beinggreater than the bending stiffness of the distal portion.
 23. The bonescrew of claim 22 wherein the proximal portion is threaded.
 24. The bonescrew of claim 23 wherein the threaded distal portion has a distalthread pitch and the threaded proximal portion has a proximal threadpitch, the proximal thread pitch being different from the distal threadpitch.
 25. The bone screw of claim 22 wherein at least the distalportion comprises a polymeric material.
 26. A method of inserting ascrew into a bone, comprising: selecting a bone screw comprising anelongate body having a distal portion, a mid-portion and a proximalportion spaced longitudinally relative to a longitudinal axis, thedistal portion having a helical thread formed thereon, the thread havinga major diameter and a minor diameter, the mid-portion having anon-threaded outer surface with an outer diameter, the mid-portion outerdiameter being equal to or greater than the thread major diameter;forming a curved tunnel in a bone; and turning the threaded distalportion of the bone screw into the tunnel so that the threaded distalportion bends to follow the curve of the tunnel.
 27. The method of claim26 wherein the bone screw further comprises a helical thread formed onthe proximal portion, the method further comprising: engaging theproximal screw threads with a proximal portion of the bone.
 28. A methodof inserting a screw into a bone comprising: selecting a bone screw froma plurality of bone screws including a first bone screw comprising anelongate body having a distal portion, a mid-portion and a proximalportion spaced longitudinally relative to a longitudinal axis, thedistal portion having a helical thread formed thereon, the thread havinga minor diameter, a pitch and a major diameter; and a second bone screwcomprising an elongate body having a distal portion, a mid-portion and aproximal portion spaced longitudinally relative to a longitudinal axis,the distal portion having a helical thread formed thereon, the threadhaving a minor diameter, a pitch and a major diameter; the second bonescrew thread minor diameter being equal to the first bone screw threadminor diameter, the second bone screw thread pitch being equal to thefirst bone screw thread pitch, and the second bone screw thread majordiameter being greater than the first bone screw thread major diameter;forming a tunnel in a bone; turning the selected bone screw into thetunnel.
 29. The method of claim 28 further comprising: after forming thetunnel in the bone, tapping a screw thread into the tunnel correspondingto the first screw thread.
 30. The method of claim 29 furthercomprising: after tapping a thread into the tunnel corresponding to thefirst screw thread, tapping a thread into the tunnel corresponding tothe second screw thread, wherein the first and second screw thread havethe same minor diameter and pitch and wherein the second screw threadhas a larger major diameter than the first screw thread.
 31. The methodof claim 30 further comprising: selecting the second bone screw andthreading it into the tapped hole.